TISSUES AND SIMPLE ORGANS. 127 



spongy tissue a third essential function, namely, the aeration of 

 the typical assimilating tissue (the palisades). The latter contains 

 numerous but narrow air-channels which are arranged about each 

 palisade- cell ; but the supply of CO 2 and the nearest centres of 

 accumulation for the liberated O are naturally to be sought for in 

 the spongy tissue, since it communicates directly with the atmos- 

 phere. Particulars will be given below (VII., Special Function). 



The teleological consideration of nature suggests that not all 

 leaves met with in nature are built ' ' bif acially ' ' and equipped 

 with anatomically different light- and shade-surfaces. Observation 

 teaches that beside the large number of flat leaves placed horizon- 

 tally there are many of cylindrical form (linear leaves), and others 

 which are flat, but not horizontal, either having the margin turned 

 toward the stem (CalUstemon, Lactuca scariola) or placed ap- 

 proximately vertical. The latter position occurs among some 

 grasses, among orchids, in Acorns, etc. From this may be deduced 

 the following : 1 



A. A "centric" type of structure with a two-sided or cylin- 

 drical evenly developed chlorophyll-bearing parenchyma is peculiar 

 to those flat leaves not horizontally placed, as many grasses, or- 

 chids, Acorus, Lactuca scariola, Callistemon, etc. (see Fig. 72). 

 Also those leaves approximately cylindrical needles, so called. 

 To the latter should also be added the green culm-like stems (hal- 

 martige /Stengel). 



B. The majority of leaves belong to the bifacial type and are 

 always flat and placed horizontally. I shall not hesitate in citing a 

 very striking example of adaptive phenomenon. The leaves of 

 Allium ursinum, Alstrwmeria, and others, in their early develop- 

 ment cause the morphologically lower surface of the leaf to be 

 turned upward by a torsion of 180 of the petiole or leaf -basis. In 

 these leaves the morphologically lower surface possesses the 

 structural arrangements for active assimilation. An analogous 

 example has been observed by SCHWENDENER in the mechanical 

 adaptation of the leaf of Gynerium argenteum. 



The same physiological significance as that of the mormal bifa- 

 cial leaf-structure also underlies the fact that in the lichens the 

 assimilating algal cells ("gonidia") are found nearest the luminous 

 side of the leaf -like thallus (see the chapter on symbiosis). 



1 DE BARY, Comparative Anatomy, page 406, et seq. 



